A method of error detection of a flight management system coupled with a guidance of an aircraft according to a flight plan, comprises the steps of: generating a first reference guidance order, monitoring the integrity of the first reference position, when the first reference position is not monitored as being dependable: invalidating the first FMS assembly and the associated guidance system, when the first reference position and the first reference trajectory are monitored as being dependable: generating a first monitoring guidance order, generating a first reference flight control, generating a first monitoring flight control, in monitoring the integrity of the first reference guidance order when the first reference guidance order is not monitored as being dependable: invalidating the first FMS assembly and the associated guidance.